FIG. 1 shows a typical circuit configuration of a conventional multi-output DC-DC converter 100 having a secondary side post regulation. The switching DC-DC converter 100 includes a power inverter 101, a power transformer 102, an output circuit 103 and a plurality of post voltage regulators 104 and 105. The power inverter 101 includes a switch circuit 111, which is typically implemented by a MOSFET switch for transferring energy received from an input DC voltage Vin to the power transformer 102 according to its on/off operations. The power inverter 101 further includes a main pulse width modulator (PWM) 112 for controlling the on/off operations of the switch circuit 111. The power transformer 102 includes a primary winding 1021 coupled to the switch circuit 111 and a secondary winding 1022 coupled to the output circuit 103 for providing an electric isolation between the input terminal and the output terminal of the DC-DC converter, wherein the power transformer 102 is configured to receive an input DC voltage Vin from the primary winding 1021 and transfer energy across the secondary winding 1022 according to the open and close of the switch circuit 111. The output circuit 103 which is made up of rectifying diodes 1031 and 1032 is coupled to the secondary winding 1022 for receiving energy from the secondary winding 1022 and providing a predetermined output voltage Vout, for example, 12 volts, to a load (not shown). Also, each individual post voltage regulator 104,105 is tapped to the secondary winding 1022 for providing a lower regulated DC voltage, for example, 5 volts or 3.3 volts.
FIG. 2(A) shows a typical circuit representation of a post voltage regulator of FIG. 1. The post voltage regulator of FIG. 2(A) includes a current blocking circuit 141, a synchronous rectifier 142, an output filter 143, a feedback circuit 144, a blocking controller 145, and a gate driver 146. The current blocking circuit 141, which is implemented by a MOS transistor, is coupled to the secondary winding 1022 of the power transformer 102 shown in FIG. 1 for blocking the transfer of current (and energy) from the secondary winding 1022 to the output stage of the post voltage regulator via its inherent body diode 1411 during the blocking time interval. The synchronous rectifier 142 which is implemented by a transistor switch is coupled to the current blocking circuit 141 for rectifying a square wave AC voltage induced on the secondary winding 1022 of the power transformer 102 into a rectified DC voltage. The output filter 143 is made up of by a choke coil L100 and a smoothing capacitor C100 for smoothing the rectified DC voltage of the post voltage regulator so as to provide a constant DC voltage at its output terminal. The feedback circuit 144 is coupled to the output terminal of the output filter 143 for calculating a difference between a fractional output voltage of the post voltage regulator and a reference voltage and generating a feedback amount dependent on the output voltage of the post voltage regulator accordingly. The blocking controller 145 is coupled to the gate terminal of the current blocking circuit 141 for controlling the blocking time interval of the current blocking circuit 141 according to the feedback amount, thereby making fine adjustments to the output voltage of the post voltage regulator. The gate driver 146 is coupled to the gate terminal of the synchronous rectifier 142 for driving the synchronous rectifier 142 to achieve synchronous rectification. Further, the post voltage regulator includes a reverse current protection diode D100 for preventing a reverse current from flowing through the current blocking circuit 141.
FIG. 2(B) shows another typical circuit representation of a post voltage regulator of FIG. 1. The post voltage regulator of FIG. 2(B) includes a gate driver 241, a RC network (Rt,Ct), a voltage-controlled current source 242, a synchronous rectifier switch 243, an output filter 244, and a feedback circuit 245. The composition and principle of the output filter 244 and the feedback circuit 245 are similar to the composition and the principle of their counterparts of FIG. 2(A), and their explanations are omitted herein for simplicity. In FIG. 2(B), the gate driver 241 provides a series of control pulse signals to turn the synchronous rectifier switch 243 on and off in a controlled duty cycle, so that the magnitude of the output voltage of the post voltage regulator can be adjusted for the compensation for the variation of the output voltage. The voltage-controlled current source 242 and the RC network (Rt,Ct) form a ramp signal generator 250, in which the capacitor Ct is charged with an imposed time constant for producing a time-varying ramp voltage. The voltage-controlled current source 242 is used to fine tune the charging rate of the capacitor Ct. The time-varying ramp voltage is provided to the gate driver 241 for calculation with a feedback signal derived from an error amplifier. (not shown) of the feedback circuit 245 in order to generate control pulse signals for controlling the switching duty cycle of the synchronous rectifier switch 243. The diode Dr is configured to make sure that when the ramp signal generator 250 is activated, the voltage across the capacitor Ct can be discharged quickly. In addition, the diode Df functions as a freewheeling diode and is coupled to the choke coil of the output filter 244 for providing a current condition path for the release of the energy stored in the choke coil when the load voltage decays to zero.
However, the prior art multi-output DC-DC converter discussed hereinbefore suffers several disadvantages needing to be immediately addressed. First, when isolation is employed in a DC-DC converter, the input voltage is typically switched on and off at a high frequency, and provided to a power transformer, which provides the input/output isolation and the suitable voltage conversion. However, because the input voltage is switched at the high frequency, the output voltage and current typically cannot be directly provided to a load in a regulated manner. Thus, an inductor is generally required in the energy conversion to act as a current filter. The size and value of the inductor are often critical to meeting the performance specifications. A large inductance volume normally reduces the power density of the converter. Further, because inductors with large inductance values have low slew rates, the response time of the converter to load current disturbances is slowed down. Accordingly, smaller inductance volumes and values are desirable.
Secondly, isolated DC-DC converters typically operate with at least some amount of dead time. Dead time indicates the time lag for preventing two switch elements to turn on at the same time. During dead time operation, a rectification current is set to flow through the body diodes of the switch elements. Hence, dead time loss (that is, the body diode conduction loss) would yield a great power loss and deteriorate the overall power efficiency for the DC-DC converter.
It is therefore inclined to develop a multi-output DC-DC converter with an increased power efficiency and improved power density.